The difference between "pellets" and "layer pellets" is calcium content. Laying mash/pellets has a higher calcium content.
I always feed pellets. I think chickens waste way too much feed in mash form.
I don't buy "sacks" of oyster shell. TSC offers much smaller quantities.
I free feed my chickens from a hanging feeder inside the chicken house. When I fill up the feeder, I'll toss in a handful of oyster shell.
I don't refill the feeder until it's empty and I don't add oyster shell until it's all gone.
They get "some" oyster shell and I don't waste a bunch of oyster shell.
